1. Field of the Invention
This invention relates to a safety valve for use in the production of oil and gas. It especially relates to a safety valve set in a tubing string of a well drilled in an offshore or water-covered area. In such situations the safety valve is usually set in the tubing string below the mudline. Then, if the tubing string is broken by any means such as a ship or barge which may be adrift in a storm, the subsea safety valve will shut off the flow of fluids through the tubing string and prevent a "runaway" well which would otherwise occur.
2. Prior Art
There are many subsurface safety valves which are commercially available and even many others that have been described in the patents and literature. Some of these valves are operated or controlled from the surface, normally by a long slender tube which supplies fluid under pressure to hold the valve in an open position. If the control fluid supply should be disrupted, the valve automatically closes. Another type downhole safety valve is that group which are self-contained, they usually depend upon a pressure drop through the valve as the actuating medium or force. If the pressure drop through the valve becomes excessive, the valve is forced to a closed position. The present invention belongs in the self-contained group but does not depend upon a pressure drop through the valve; in fact, an object of this invention is to maintain as low a pressure drop as possible.
Perhaps the closest prior art is U.S. Pat. No. 1,961,280, W. J. Crites et al., for "Method and Apparatus for Controlling Oil Wells," issued June 5, 1934. That patent is not, in a strict sense, a safety control valve but rather is a temperature-sensitive device at the bottom of a wellbore so that the well may be produced at a rate and/or in such a manner that the least amount of gas will be produced. In the device of that patent, an increase in temperature causes an increase in the flow of oil and gas and, likewise, a decrease in temperature causes a decrease in the flow of oil and gas. This is the opposite of what occurs in the present invention.